Depending on the position of the IS element at the time you shoot (this is what I believe to be the cause), you sometimes get a miniscule darkening of the two edges where the coverage suffers. This also is easily corrected, and is mostly unnoticable.

This may simply be slight vignetting and likely affects only two corners because the lens is not perfectly centered. Here's vignetting from using stacked filters on my FZ150 showing two corners vignette slightly more than the other two - I'd suspect the same is true for your FZ200: